The present invention relates generally to the field of boiler construction and in particular to a new method for making near drum boiler tubes more resistant to corrosion or erosion.
Smaller, low pressure industrial boilers typically use an additional bank of heat exchanger tubes called the boiler bank or steam generating bank. This additional bank is needed when the heat transfer surface in a furnace may not be sufficient to generate enough saturated steam for the particular end use desired. This boiler bank is typically composed of a steam drum on top, a second mud drum on the bottom, and a series of bent connecting tubes. Subcooled water travels down the tubes into the lower drum. The water is then distributed to other tubes where it is partially converted to steam and returned to the steam dream. The lower drum is often called the mud drum because sediments found in boiler water tend to settle out and collect in this drum. A construction of a boiler bank is illustrated and described in Chapter 1, page 1-8 of Steam/its generation and use, 40th Ed, Stultz and Kitto, 1992.
Steam generating bank tubes in recovery boilers and bio-mass fired power boilers are particularly susceptible to near drum corrosion/erosion, and in particular, tube wall thinning near the lower drum of the boiler's generating bank. Generating bank tubes are inserted and rolled into the steam and mud drums. The near drum corrosion/erosion takes place on the generating bank tube next to the drum surface. The near drum wastage from corrosion/erosion requires that the tubes in the generating bank are replaced in a periodic fashion to prevent tube leaks in the thinned areas of the wastage.
A number of remedies have been attempted to reduce the wastage rate from near drum corrosion/erosion. U.S. Pat. No. 6,495,268 and U.S. Pat. No. 6,800,149 entitled “Tapered Corrosion Protection of Tubes at Mud Drum Location”, remedies this problem under certain conditions. Laser cladding is used to produce a corrosion resistant cladding layer, which advantageously tapers along a length of the end portion of the tube. A protective layer of stainless steel powder is fused to the surface of each tube's outer diameter using the laser beam. This manufacturing process protects the portion of the tube that penetrates the lower steam drum.
Chromizing of the tubing near the drums also has been attempted with limited success. Ceramic sleeves, tube shields, metal spray coatings in the affected area and thicker tubes have all been attempted with limited success as well.
A variety of other corrosion resistant technologies are described in the background section of U.S. Pat. No. 6,495,268 and U.S. Pat. No. 6,800,149, and are hereby incorporated by reference.
U.S. Pat. No. 4,658,761 discloses in the background section that one expensive solution to corrosion of boiler tubes is an expensive technique whereby corrosion resistant metal alloys are co-extruded over mild steel tubes.
U.S. Pat. No. 4,463,061 discloses that it has been proposed to provide a composite boiler tube having a surface layer and an inner layer in which outer layer there is afforded an improved high temperature corrosion resistance, as shown in a prior art reference entitled “Co-Extruded Tubes Improve Resistance to Fuel Ash Corrosion in U.K. Utility Boilers” described by T. Flatly et al in “Material Performance Vol. 20 No. 5” dated May, 1981. However, the composite boiler tube of the reference is still insufficient in the points of high temperature corrosion resistance and creep strength in a case where the temperature of the outer surface layer of a boiler tube becomes about 700 degrees Celsius or above.
The two types of co-extruded tubing that are commercially available for use in boiler tubing today include 2.500″ OD×0.257″ wall thickness carbon steel base tubing with a co-extruded outer layer of either Nickel-Iron-Chromium alloy with additions of Molybdenum and Copper under the tradename INCONEL 825 or stainless steel (SA213TP304). These are used in areas of the high corrosion rates to reduce wastage of the boiler tubing.
A more flexible solution that works under various conditions is still needed for decreasing the corrosion/erosion rate of generating bank tubes in the region near the mud or steam drums. A solution is also needed for a Iternatively providing a way to protect the entire length of the generating bank (in the case of making the entire tube out of the co-extruded tube) or just the portion near the drum can be protected (in the case of using just a small length of the co-extruded tubing near the drum).